This invention is concerned with hot melt adhesives comprising a polymer or copolymer of glycolic acid. In particular, this invention provides an improved hot melt adhesive comprising a polymer or copolymer of glycolic acid and a minor amount of silica. Such compositions have been found to have unexpectedly good thermal stability.
Glycolic acid is a bi-functional hydroxyacid which can be made by the hydrolysis of cloroacetic acid or by the oxidation of ethylene glycol with dilute nitric acid. Being bi-functional glycolic acid can be polymerized or copolymerized rather easily to form high molecular weight polymers. U.S. Pat. No. 2,676,945 describes glycolic acid polymerization by polycondensation carried out in the solid state at about 220.degree. C. under reduced pressure.
Glycolic acid polymers, at times referred to as either "polyglycolic acid" or "polyglycolides," have a variety of end uses. For instance, being soluble in physiological fluids they have been used as an absorbable suturing material. In order to enhance or modify the physical properties of polyglycolides they have been copolymerized with various other functional compounds. In particular, polybasic acids and polyhydric alcohols have been copolymerized with polyglycolide to form polyesters often referred to as "alkyd resins." In the molten state these resins are usually quite viscous. Accordingly, glycolic acid homo- and copolymers have been suggested for use in hot melt adhesive compositions.
U.S. patent applications Ser. Nos. 812,887 filed July 5, 1977 and Ser. No. 826,491 filed Aug. 22, 1977 describe glycolic acid copolymers which are useful in hot melt adhesive compositions. Ser. No. 812,887 describes copolymers of lactones and glycolic acid or a glycolic acid homopolymers. Ser. No. 826,941 describes terpolymers of either glycolic acid or a homopolymer of glycolic acid, a dihydric alcohol, and a dibasic acid. While these materials can be used to produce acceptable hot melt adhesive compositions, it has been found that at elevated temperatures the compositions lose stability and become unacceptable for some applications.
The loss of stability at elevated temperatures weakens the bond strength of the adhesives and allows the bonded materials to separate or "creep". Thus, the resistance of adhesive compositions to the loss of stability is called "creep resistance." The corrugated paper manufacturing industry has fairly definite limitations on the acceptable creep resistance of hot melt adhesives. Surprisingly, hot melt adhesives comprising glycolic acid polymers have generally failed the corrugated paper manufacturing specifications unless annealed after application or unless allowed to age before application. However, neither the annealing nor the aging steps are desirable if existing equipment is to be used. An alternative method of imparting increased creep resistance to hot melt adhesives comprising a glycolic acid polymer is essential to the commercial acceptance of these adhesives.
This invention concerns the discovery that, if minor amounts of silica are incorporated into hot melt adhesive compositions comprising a glycolic acid polymer, the creep resistance of the adhesive compositions is significantly increased. In fact, adhesive compositions which failed for use in the manufacture of corrugated paper have been improved by the incorporation of a minor amount of silica to the point where they easily pass the creep resistance requirements of that industry. Silica has been suggested for use as a filler in hot melt adhesives. Suggested amounts range from 1% to 150% by weight. For instance, U.S. Pat. No. 4,031,058 describes hot melt sealants comprising a blend of a partially neutralized random copolymer ethylene/methyl acrylate/maleic acid monoethyl ester, with a tackifying agent, a plasticizer, and a filler. The filler comprises from about 10% to 50% by weight of the composition and may be carbon black, calcium carbonate, titanium dioxide, clays, or silica. U.S. Pat. No. 3,657,389 describes hot melt adhesive compositions comprising polyesters blended with polyolefins or vinyl polymers. The crystalline polyesters are preferred to amorphous polyesters since the crystalline polyesters are useful at higher temperatures. These compositions optionally include fillers to improve temperature resistance. Suitable fillers include silica, alumina, or calcium carbonate, all of which affect the degree of crystallinity and crystal size of the polyesters.